1. Field of the Invention
The present invention relates to a method of forming patterns, which is usable in a production process of semiconductors, such as ICs, manufacturing of circuit boards for liquid crystal displays or thermal heads, and other lithography processes of photofabrication. More specifically, the invention is concerned with a method of forming patters by using a resist composition for negative development, a protective film composition for protecting a film formed from the resist composition and a negative developer in the lithography process performing light exposure by means of immersion-type projection exposure apparatus.
2. Description of the Related Art
As semiconductor devices have become increasingly finer, progress has been made in developing exposure light sources of shorter wavelengths and projection lenses having higher numerical apertures (higher NAs). Up to now, steppers using as light sources ArF excimer laser with a wavelength of 193 nm and having NA of 1.35 have been developed. As generally well known, relationships of these factors to resolution and depth of focus can be given by the following expressions;(Resolution)=k1(λ/NA)(Depth of Focus)=±k2·λ/NA2 where λ is the wavelength of an exposure light source, NA is the numerical aperture of a projection lens, and k1 and k2 are coefficients concerning a process.
As an art of heightening the resolution in an optical microscope, the method of filling the space between a projection lens and a test specimen with a liquid having a high refractive index (hereinafter referred to as an immersion liquid or an immersion medium), or the so-called immersion method, has hitherto been known.
This “immersion effect” can be explained as follows. When the immersion method is applied, the foregoing resolution and depth of focus can be given by the following expressions;(Resolution)=k1·(λ0/n)/NA0 (Focal depth)=±k2(λ0/n)/NA02 where λ0 is the wavelength of an exposure light source in the air, n is the refractive index of an immersion liquid relative to the air and NA0 is equal to sin θ when the convergent half angle of incident rays is represented by θ. That is to say, the effect of immersion is equivalent to the use of exposure light with a 1/n wavelength. In other words, application of the immersion method to a projection optical system having the same NA value can multiply the focal depth by a factor of n.
This art is effective on all shapes of patterns, and besides, it can be used in combination with super-resolution techniques under study at present, such as a phase-shift method and an off-axis illumination method.
Examples of apparatus utilizing this effect for transfer of fine circuit patterns in semiconductor devices are disclosed in Patent Document 1 (JP-A-57-153433) and Patent Document 2 (JP-A-7-220990), but these documents have no description of resist suitable for immersion lithography.
Recent progress of immersion lithography is reported, e.g., in Non-patent Document 1 (Proc. SPIE, 4688, 11 (2002)), Non-patent Document 2 (J. Vac. Sci. Technol., B 17 (1999)), Patent Document 3 (WO 2004/077158) and Patent Document 4 (WO 2004/068242). In the case where ArF excimer laser is used as a light source, purified water (refractive index at 193 nm: 1.44) is the most promising immersion liquid in point of not only handling safety but also transmittance and refractive index at 193 nm, and actually applied in mass production too. On the other hand, it is known that immersion exposure using a medium of a higher refractive index as immersion liquid can offer higher resolution (Non-patent Document 3: Nikkei Microdevices, April in 2004).
For the purpose of supplementing the sensitivity of resist, which has been reduced by light absorption from the resist for KrF excimer laser (248 nm) onward, the image formation method referred to as a chemical amplification technique has been adopted as a method of patterning resist. To illustrate an image formation method utilizing chemical amplification by a positive-working case, images are formed in such a process that exposure is performed to cause decomposition of an acid generator in the exposed areas, thereby generating an acid, and conversion of alkali-insoluble groups into an alkali-soluble groups by utilizing the acid generated as a reaction catalyst is caused by bake after exposure (PEB: Post Exposure Bake) to allow the removal of exposed areas by an alkaline developer.
Since application of immersion lithography to chemical amplification resist brings the resist layer into contact with an immersion liquid at the time of exposure, it is pointed out that the resist layer suffers degradation and ingredients having adverse effects on the immersion liquid are oozed from the resist layer. More specifically, Patent Document 4 (WO 2004/068242) describes cases where the resists aimed at ArF exposure suffer changes in resist performance by immersion in water before and after the exposure, and indicates that such a phenomenon is a problem in immersion lithography.
As a solution to avoidance of this problem, a method of keeping resist from direct contact with water by providing a protective film (hereinafter referred to as a topcoat or an overcoat too) between the resist and a lens is known (e.g., in Patent Documents 5 to 7: WO 2004/074937, WO 2005/019937, and JP-A-2005-109146).
In such a method, it is required for the topcoat to have no solubility in immersion liquid, transparency to light from an exposure light source and a property of not causing intermixing with a resist layer and ensuring stable coating on the top of a resist layer, and besides, from the viewpoint of pattern formation by as-is utilization of an existing process, it is preferable that the topcoat has properties of easily dissolving in an alkaline aqueous solution as a developer and allowing removal simultaneous with removal of a resist layer by development.
However, topcoat-utilized image formation methods hitherto known fail to fully satisfy performance capabilities required for immersion lithography. For example, the pattern formation methods using topcoats hitherto known cause problems of development defects appearing after development and degradation in line edge roughness, and they are in need of improvements.
At present, an aqueous alkali developer containing 2.38 mass % of TMAH (tetramethylammonium hydroxide) has broad use as developers for g-ray, i-ray, KrF, ArF, EB and EUV lithographic processes.
As a developer other than the aqueous alkali developer, the developer used for performing development by dissolving exposed areas of a resist material reduced in molecular weight through cleavage of its polymer chains upon irradiation with radiation, and that characterized by having at least two functional groups of more than one kind chosen from an acetic acid group, a ketone group, an ether group or a phenyl group and a molecular weight of 150 or above, is disclosed, e.g., in Patent Document 8 (JP-A-2006-227174). In addition, the developers used for performing development by dissolving exposed areas of specified resist materials containing fluorine atoms and chosen from supercritical fluids, halogenated organic solvents or non-halogenated organic solvents are disclosed in Patent Document 9 (JP-T-2002-525683, the term “JP-T” as used herein means a published Japanese translation of a PCT patent application) and Patent Document 10 (JP-T-2005-533907).
However, as the semiconductor devices become finer, it becomes exceedingly difficult to find combinations of, e.g., a resist composition, a developer and a protective film (topcoat) composition appropriate to formation of patterns with synthetically good quality, and it is required to find pattern formation methods which can achieve, e.g., reduction in development defects appearing after development and satisfactory line edge roughness and can be applied suitably to topcoat-utilized immersion lithography.